Anton Kerner von Marilaun.

The natural history of plants, their forms, growth, reproduction, and distribution: from the German of Anton Kerner von Marilaun (Volume 2) online

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occurs. Two or three days later, a legitimate crossing may take place, for, by this
time the anthers of the Purple Willow will have protruded from the staminate
flowers and opened widely, and abundance of pollen will be afforded to insects.
These are not slow to visit the now accessible flowers, and they remove some of the
pollen and transfer it to the stigmas of the same species which are still capable of
being fertilized. Thus at the commencement of flowering hybridization is alone
possible, and legitimate cross-fertilization cannot take place till some time later,



SIGNIFICANCE OF DICHOGAMY. 315

in consequence of the dichogamy of these Willows. This obviously applies to all
other Willows, and generally to all dioscious plants whose flowers are incompletely
protogynous.

In order to show that the same processes obtain in monoecious plants, I would
ask the reader to accompany me to the edge of a moor where numerous inonn -clous
ISedges (Carex) form the chief constituents of the vegetation. Widely different
Ispecies grow in varied profusion side by side. Here at the margin of a dark pool
Carex acutiformis, filiformis, riparia, vesicaria, paniculata, there, on the marshy
stretch close by, Carex flava, canescens, glauca, Homschuchiana, and many others.
These Sedges do not all blossom at the same time; one ceases to flower just when
another is in its prime, and when, in a third sort, the flowers have just begun to
fade. All monoecious Sedges are protogynous. The stigmas have been ripe 2-3 days,
and have protruded far beyond their subtending bracts, so that it would seem
natural that the pollen, wafted by the wind, would remain attached to them. But
the anthers of the staminate flowers of the same species have not yet opened. It
is evident then that the stigmas must be pollinated during the first and second day
with pollen from other species which blossom earlier, for since the anthers of these
earlier species are already open, each gust of wind will shake out their pollen and
blow it over the moor, pollinating everything which is capable of being pollinated.
The pollen of the same plants (afterwards shed from the staminate flowers above
and close to the mature stigmas) can only be received in the second place on account
of its later arrival. Thus, we see that incomplete dichogamy promotes hybrid-
ization- in the first place, and then, only later, a legitimate cross-fertilization in
plants with monoecious flowers.

It is well known that all the plants of a species growing under similar external
conditions do not blossom on the same day, and this fact is worth noticing in so far
as it might be thought possible for the earlier plants of a species to provide pollen
for the stigmas of later plants. This is certainly often the case, but it is also certain
that the stigmas of the very earliest plant of a protogynous species can only be, and
actually are, fertilized with pollen from another species which flowers still earlier;
thus the conclusion already arrived at must remain unaltered.

It may be taken for granted, since plants with pseudo-hermaphrodite flowers
behave exactly like dioecious and monoecious flowers in the manner of the transfer
of their pollen, that their dichogamy has the same significance. The spikes of ]
belonging to the group Lapathum, viz. Rumex alpinus, nemorosus, and obtwfol
bear principally pseudo-hermaphrodite flowers, which are some of them male,
female, and besides these a few true hermaphrodite flowers. In any one plant the
development of the stigmas is always considerably in advance of that of the anthers.
The stigmas are ripe whilst the anthers are still closed. Under these urcnnufa
the first flowers of a plant, whether pseudo- or truly hermaphrod! to can only rec
pollen from other plants which have been in bloom for several days and wh<
Lhisced versatile anthers have been robbed of their pollen by the wmd I ma,
further be taken for granted that any hundred plants of Rumex obtwfohus,



316 THE CROSSING OF FLOWERS.






growing together in a clump, will not all blossom together, and consequently
innumerable crossings take place between the flowers of the individuals of this
same species. The first ripe stigmas of the earliest plants of Rumex obtusifolius
within an hour's walk can only receive their pollen during the first two days from
other species of Dock, and therefore, when they first blossom, hybridization only
can occur. It has already been stated that plants of Marjoram (Origanum vulgare,
a Labiate), which bear pseudo-hermaphrodite female flowers, blossom fully eight days
before those with truly hermaphrodite flowers. To this we might add that the
plants which blossom first in any given district cannot obtain pollen from the same
species, and that consequently, if the stigmas are, nevertheless, pollinated by insects :
the pollen must have been obtained from some other species. In Compositse,
whose capitula contain both truly hermaphrodite and pseudo - hermaphrodite
female flowers, the latter always mature some days before the former, and con-
sequently the pioneer flowers in a given locality can only obtain pollen from
species which bloom still earlier, so that again hybridization occurs. In the
floral region of the Black Sea many Fleabanes grow side by side (Inula Oculus-
Christi, ensifolia, Germanica, salicina, &c.), and in the summer they blossom
in definite succession, so that one species always begins to fade when another is
in its prime. Each capitulum of these Inulas consists of tongue-shaped pseudo-
hermaphrodite female flowers on the circumference and tubular hermaphrodite
flowers in the centre. The former unfold earlier than the latter, and for each
species there is a certain period, if only two days, when the pollen, brought by
insects to the stigmas of the pistillate flowers on the circumference, can only have
been obtained from another species, since their own pollen is not obtainable. Many
other examples of the same .kind might be quoted, all pointing to the fact that
hybridization at the commencement of flowering and the subsequent legitimate
crossing depend mainly on the incomplete dichogamy obtaining in these plants.

It is the same with plants which have true hermaphrodite flowers. In hetero-
styled species either the long-styled or the short-styled flowers may develop first.
The long-styled flowers of Primula Auricula and the short-styled flowers of
Primula longiftora are the earlier, consequently, the stigmas of the first long-styled
Primula plants can only be fertilized with pollen from other species. This is
often actually effected by insect-agency, and gives rise to numerous Primula hybrids.
The same thing is repeated in hermaphrodite flowers which are not heterostyled.
When a plant is protogynous, as, for example, the open-flowered Pasque-flower,
Pulsatilla patens, the earliest flowers can receive no pollen from anthers of their
own species, because not one is open, but it would be possible for them to be
provided with the pollen of other species of the same genus which inhabit the same
locality but blossom earlier. This, of course, only holds good for the commencement
of the flowering period, and only for those plants of the species which are the first
to open their flowers in a given place. At a later period of flowering legitimate
crossing will occur, because by that time the earliest plants have shed their pollen,
and it may be collected and transferred by insects. Among hermaphrodite plants'



SIGNIFICANCE OF DICHOGAMY. 317

ihere are many whose flowers are not protogynous but protandrous. Here the
stigmas of the earliest flowers of a species cannot be pollinated, because they are
.mmature and inaccessible. What, then, becomes of the pollen of these first prot-
indrous flowers ? If it is carried by the wind or by insects, as soon as it is liberated
:rom the anthers, to any stigma, that stigma must of necessity belong to another
species which has already become receptive. Towards the end of the flowering
period, the pollen usually runs short in most protandrous species, whilst the stign
rf these stragglers have only just attained maturity. They could only obtain
pollen from flowers which had not developed so far. But if these flowers are the
last in the locality, and they are protandrous, there is no more pollen to be had
from that species, and obviously they must be satisfied with pollen from some other.
Accordingly hybridization is a matter of necessity in the latest flowers of herma-
phrodite plants which are protandrous, just as it is in the earliest flowers of those
which are protogynous.

From these facts we may infer that every dichogamous plant has an opportunity
for illegitimate crossing or hybridization at the beginning or end of its flower-
ing, and that dichogamy especially incomplete dichogamy is the most important
factor in its production. Of course this does not exclude dichogamy from playing
an important part in legitimate crossing as well. On the whole, however, we can
maintain the view that the separation of the sexes by the maturation of the sexual
organs at different times leads to hybridization, whilst their separation in space
promotes legitimate crossing. The fact that the separation of the sexes in time
and space usually occur in conjunction, harmonizes with this conclusion, i.e. that
the dioecious, monoecious, and pseudo-hermaphrodite flowers, as well as those herma-
phrodite flowers whose sexual organs are separated by some little distance, are
in addition incompletely dichogamous, because by this contrivance the flowers of
any species obtain (1) the possibility of hybridization at the beginning or end of 1
flowering period, and (2) of legitimate crossing during the rest of that time.
exDlains why incomplete dichogamy is so much more frequent than complete dicho-
gamy why there are no dioecious species of plants with completely dichogamoi
flowers- and why, if one ever should occur, it would of necessity soon disappear. Let
us suppose that somewhere or other there grows a species of Willow with complc
protogynous dioecious flowers, that is to say, a species in which the female
mature first, and have ceased to be receptive before the male flowers m the same
region discharge their pollen. Hybridization only could occur m it, and
fallow plants resulting from it would all be hybrids whose form wou d no longe
agree absolutely with that of the pistilliferous plant. The species would ther
not be able to reproduce its own kind by its seed, and it would leave no descen-
dants of similar form ; in other words, it would die out.



318 THE CROSSING OF FLOWERS.




and of geitonogamy (from ydruv, a neighbour, and ydfj.os } marriage) when the tw(
flowers are on the same plant. We cannot, however, draw a sharp line betw<
the two. The offshoots of a plant, which become ultimately isolated, forming ind(
pendent plants, are, in point of origin, identical with the branches of a plant whic
remain attached. Accordingly, when a crossing occurs between flowers produced
plants that have arisen from one another by offshoots, the process is not essentially!
different from the crossing which takes place between flowers on adjacent branches.)
It is nevertheless convenient to keep the two cases distinct, on account of certain!
processes connected with the greater or lesser distances between the flowers.

Both in xenogamy and geitonogamy the transport of the pollen is effected
partly by wind and partly by flower-visiting insects. How this is carried out,
and the endless variety which exists, has been dealt with in detail in previous
chapters. Geitonogamy is not infrequently, however, brought about in other ways,
as by the pressing of mature stigmas on the liberated pollen of neighbouring flow(
or by the actual falling of pollen upon them. Since these methods of
pollination have only been incidentally touched upon, they must be described h<
somewhat more fully.

The conditions for crossing between neighbouring flowers are simple when the
flowers are crowded in heads, umbels, bunches, spikes, and the like, standing so
close together that the stigmas of one flower can easily touch the pollen-covered
anthers of another. And since this kind of crossing is actually very widespread
and is repeated in certain species with great regularity, generation after generation,
we are justified in regarding these forms of inflorescence as being particularly
associated with geitonogamy, and hi assuming that a not unimportant advantage
of a crowded inflorescence lies in the possibility of crossing between the neighbour-
ing flowers of a plant (see vol. i. p 740).

As we might expect, this particular form of crossing occurs with great frequency
in Composite, whose flowers are crowded so densely into capitula that the whole
inflorescence might be taken, at first sight, for a single flower; consequently this
extensive family of plants, which includes more than 10,000 species, will be the most
suitable in which to describe the phenomenon of geitonogamy. We will commence
with those Composites whose heads only contain "ray" or ligulate florets. The
term ray or ligulate floret is applied to florets whose corolla is tubular only at the
base, the free end being flattened and projecting outwards like a tongue or strap, as
in the Dandelion. In Prenanthes each capitulum consists of only five such ray-
florets. In each floret the long, thin style is inclosed in a tube of anthers. The
style is covered with stiff outwardly-directed bristles which are called " sweeping
hairs". When the style elongates, immediately after the opening of the flower,
these hairs sweep out the pollen which has been already shed into the interior of
the anther-tube. The long style, rendered quite yellow by the pollen it carries, now
projects from the empty tube of anthers. The two branches of the style which
bear the stigmatic surface are at first folded together, but they soon separate, and
the stigmas can then be fertilized by the aid of insects with pollen brought from



GEITONOGAMY. 319

other plants, but not with that which lies on the sweeping hairs below the stigmas.
As soon as the strap -shaped corollas begin to wither and shrivel, the two sty la r
branches diverge strongly, and twist and turn like tiny snakes sideways an-1
downwards. At the same time adjacent styles come nearer to one another, and it
is therefore natural that the stylar branches of neighbouring flowers should get
entangled. In this way the stigmas of one flower (which are still in a receptive
state) necessarily come in contact with the pollen on the sweeping hairs of another,
and pollination ensues.

The same process occurs in the flowers of the Lettuce (Lactuca), the Alpine
Sow-thistle (Mulgedium), and in Chondmtta, only here the heads contain more
florets than in the form just described. The stylar branches do not undergo snake-
like movements, but they diverge widely and roll back a little, an action altogether
sufficient to bring them into contact with the styles of neighbouring flowers and
to promote a crossing. It is worth noticing that the corollas of the ray -florets of
Prenanthes roll outwards when they begin to fade, while those of the Lettuce and
of the other Composites mentioned fold up and form a hood over the stylar
branches during crossing. The Salsify (Tragopogon), Hawkweed (Hieracium),
Crepis, Scorzonera, Hawkbit (Leontodon), Dandelion (Taraxacum), and many
other Composites, of which these plants may be regarded as typical, contain in each
head as many as 100 ray-florets arranged in spiral series (cf. fig. 222 6 , p. 112). The
strap-shaped corollas separate in the morning and fold together in the evening, and
similarly the anther-tubes and styles are inclined somewhat to the circumference
of the capitulum in the morning, but come close together and assume an upright
position in the evening. This gradual approach ultimately becomes actual contact,
and since the development of the protandrous florets proceeds from the circumfer-
ence towards the centre of the capitulum, the stigmas of the outer florets are
mature at the time when the pollen has only just been swept out of the anther-
tubes of the inner florets. The contact of the adjoining flowers, therefore, neces-
sarily leads to cross-pollination. The fact that the corollas of the ray-floret*
any capitulum are of unequal length (fig. 222*, p. 112) has also a close bearing on
this process. If they were all equally long this contact and crossing would
impossible, for division walls would be interposed between the styles of the
and inner florets. But the inner corollas are just short enough to allow the
to touch one another. In many of these plants, e.g. in the Salsify (Tragopogon),
geitonogamy is also assisted by the arrangement of the flowers in each capitulum,
each flower of an outer row being placed exactly between two of the nex
series. When the capitulum closes, the two curved stylar branches of an
floret, with their exposed stigmatic surfaces, become apphed to the pollens
styles of the inner flowers immediately to right and left m front of



320



THE CROSSING OF FLOWERS.



contain but few florets; those of Eupatorium cannabinum have five, which open one
after another in the course of 5-8 days. Younger and older flowers are therefore
always close together. The styles are rather different from those of other Composites,
being divided almost half-way down into two long threadlike branches which
bear the stigmatic tissue only on their lower portions. The rest of the branch is
thickly studded with short bristles, the aforesaid sweeping hairs. The styles are
parallel and folded together as long as they are inclosed in the anther-tube (see
fig. 294 2 ), and they remain closed for some time after they have elongated and




Fig. 294. Geitonogamy with adherent pollen.

1 Crossing of the stylar branches of neighbouring florets in the capitulum of Eupatorium cannabinum. 2 Longitudinal section
through the upper part of a floret of Eupatorium; the two stylar branches are parallel and inclosed by the anther-tube,
which is again surrounded by the corolla-tube. Umbel of Chcerophyllum aromaticum; the truly hermaphrodite flowers
are open, the pseudo-hermaphrodite male flowers are still closed. * The same umbel; the true hermaphrodite flowers
ha lost their pollen; the male flowers are now open, and their anthers drop pollen on the stigmas of the hermaphrodite
flowers. All the figures somewhat enlarged.

pushed their way above the anthers. During the elongation the sweeping hairs
brush the pollen from the anther-tube, and it then adheres in abundance to the
outer side of each style-branch. This condition, however, does not continue for long.
The stylar branches soon diverge at an angle of 40-50. The branches of adjacent
styles now cross one another like rapiers, and when the pollen is detached from the
sweeping hairs it falls on to the mature stigmatic tissue. The closed pollen-covered
stylar branches, as they emerge from the anther-tube, come in contact with the
divergent branches of older neighbouring florets, and thus the pollen is transferred
to the stigmatic tissue of the latter.



GEITONOQAMY. 321

The capitula of the Colt's-foot (Tuasilago) and of the Marigold (Calendula)
contain two kinds of florets. In the centre are pseudo-hermaphrodite male flowers,
whilst true pistillate ray-florets form the fringe of the capitulum. The latter open
earlier than the disc-florets, and therefore at first can only be fertilized with pollen
from other capitula which are further advanced. But soon the pollen is pushed out
of the disc-florets of the same capitulum, and is deposited in a small clump at the
top of the anther- tube. This pollen is conducted to the stigmas of the neighbouring
ray-florets by different methods in the two genera mentioned. In the Colt's-foot
the numerous ray-florets at the periphery are expanded horizontally during the
daytime, but towards evening they fold up, and in this way, as they bend over the
tubular florets, contact with their clumps of pollen is unavoidable. The pollen is
transferred to the ray-florets, and when the capitula open again next morning, and
the ray-florets bend outwards, the adherent pollen is freed, and slips down to the
ripe stigmas at the base of the corolla. The process is far simpler in the Marigold.
The stylar branches of the ray-florets are bent inwards over the adjoining disc-
florets while the latter are still closed. When they open, and the pollen is swept
out of their anther- tubes, it of course passes inevitably to the stigmas of the neigh-
bouring ray-florets which are situated just above.

The Golden-rod (Solidago), Aster (Aster), and many other Composites classed
together in the group of the Asteroideae, closely resemble the Colt's-foot and Mari-
gold in outward appearance, but their sexes are differently arranged. The tubular
disc-florets are all truly hermaphrodite, and the outer ray-florets are truly pistillate.
The latter mature first, and are adapted to hybridization, as we have already
remarked. Two days later the hermaphrodite flowers of the disc open those
towards the circumference being the first. Their pollen is pushed out, and mean-
while the flowers bend slightly outwards, so that the pollen lying on the anther-
tubes in the form of small clumps either comes into direct contact with the ripe
stigmas of the marginal ray-florets or falls on to them from a short distance.

In very many Composites the capitulum contains only hermaphrodite flowers
with tubular corollas. The development of the flowers again proceeds from the
circumference towards the centre of the capitulum, and in each flower, soon aft
the corolla has opened, the pollen is swept and pushed out of the anther-tube by
the sweeping hairs or warts on the outer side of the style. The pollen forms a
small clump at the mouth of the anther-tube, but does not retain this posil
The two stylar branches which have hitherto been folded together (their oute
surface being coated with pollen) soon separate and often bend ba*k in a curve
as to expose their ripe stigmatic surfaces. The pollen is thus for the mos
detached in small crumbling balls which simply tumble down. In tins wa



to to , n to



VOL. II.



322 THE CROSSING OF FLOWERS.

capitulum are of unequal length. The marginal florets are rather shorter than the
central ones, so that the stylar branches of the former are lower than those of the
latter. But this is not enough to bring the pollen which has fallen from the higher
stylar branches on to the stigmatic tissue of these older lower ones since the lower
are situated rather nearer the circumference of the capitulum, and it is therefore
necessary that the pollen-bearing styles should incline outwards if their pollen is
to reach its proper destination. This is what actually happens. The originally
straight and erect styles bend outwards at an angle of 70-90, even before their
branches have separated, and while they yet retain the pollen which they have
collected from the anther-tubes. When it is thrown off, it thus unavoidably reaches
the lower stigmas of the older flowers. Or sometimes it happens that the divergent
stylar branches of the younger flowers with attached pollen come into direct con-
tact with the stylar branches of older flowers, and that geitonogamy is effected in
this way.

Numerous other Composites whose capitula are composed entirely of tubular
hermaphrodite flowers exhibit the same processes as Homogyne, which has been
chosen here as a type. The Wormwoods of mountain heights, e.g. Artemisia Mutel-
lina and spicata, exhibit a slight deviation. In them the central florets are raised
above the marginal ones, not only by their greater length, but because the receptacle
on which they stand is considerably arched. Obviously the florets at the top of the
dome will stand higher than those round its circumference. In very many Com-
posites (e.g. in Doronicum glaciale and scorpioides, in Senecio cordatus, in Telelda,
Buphthalmum, Anthemis, and Matricaria), the receptacle is at first flat or but
slightly arched; but during the flowering period it rises up so much that it assumes
the form of a hemisphere, or even of a cone. This elevation in Doromcum-capitula,
for example, amounts to 1 cm., and it is relatively even more in species of Anthemis
and Matricaria. The immediate consequence of this change in the receptacle is of
course an alteration in the direction of the flowers which stand on it. Flowers
which stood erect on the receptacle of the capitulum when it first opened, assume
later an almost horizontal position. But the most remarkable thing is that these



Online LibraryAnton Kerner von MarilaunThe natural history of plants, their forms, growth, reproduction, and distribution: from the German of Anton Kerner von Marilaun (Volume 2) → online text (page 39 of 128)